Non-natural α-amino acids are a major type of amino acids which are different from 22 types of natural α-amino acids capable of being synthesized by organisms themselves. Non-natural α-amino acids have important bioactivity and physiological effects, and are applied widely in the synthesis of compounds such as polypeptides, chiral drugs and alkaloid. α-aminobutyric acid is a non-natural amino acid for inhibiting the neural information transmission of the human body, and has activity for enhancing glucose phosphatases and the effect of promoting brain cell metabolism. The α-aminobutyric acid is also an important chemical material and medical intermediate, and has been applied widely in the synthesis of drugs, such as the synthesis of the antituberculosis drug ethambutol hydrochloride and the antiepileptic drug levetiracetam, and the market is huge.
Synthesis methods for the α-aminobutyric acid mainly include three types, i.e. chemical synthesis methods, an enzymatic resolution method and an enzymatic transformation method. The chemical synthesis methods include desulphurization reaction, ammonification hydrolysis reaction, ketobutyric acid reduction, etc. Although chemical synthesis is easy to operate, reaction conditions are usually harsh and byproducts can be produced easily, and sometimes, a large quantity of organic solvents harmful to the environment needs to be utilized. For example, Jeffery E. A. et al. utilized an electrochemical method to prepare the α-aminobutyric acid, the yield was only 48 percent, and moreover, the byproduct glutamic acid existed. By contrast, a microorganism method for preparing the α-aminobutyric acid has the advantages of high specificity, mild condition, environment-friendliness, etc. Furthermore, with the development of the genetic engineering technology, the metabolic pathways of constructing recombinant microorganisms for synthesizing non-natural amino acids have been accomplished. At present, the preparation of the α-aminobutyric acid by the microorganism method is mainly based on an extracellular enzymatic transformation method, which includes carrying out enzymatic resolution preparation on racemic α-aminobutyric acid and carrying out catalytic preparation with 2-ketobutyric acid as a material by means of dehydrogenase or transaminase.
In a previous research, the inventor adopted a one-step method to prepare the α-aminobutyric acid with the bulk chemical L-threonine as a cheap substrate by means of an enzyme system composed of L-threonine deaminase, L-amino acid dehydrogenase and coenzyme regeneration system. In the process of enzymatic transformation, it was discovered that the amount of the L-threonine deaminase had to be controlled accurately, and otherwise the accumulation of an intermediate product ketobutyric acid would be caused to inhibit the transformation from the ketobutyric acid to the α-aminobutyric acid, leading to the interruption of the production of the α-aminobutyric acid by enzymatic transformation. Moreover, utilizing an enzymatic transformation system to carry out the production of the α-aminobutyric acid requires cell disruption to be carried out on three types of enzyme-producing recombinant bacteria, the process is complex, and the cost is high; furthermore, the inactivation of enzymes in the process of transformation affects the stability of transformation; and in addition, due to the loss of the cofactor, constant exogenous addition is required, further increasing the production cost of the α-aminobutyric acid. Therefore, it is necessary to find a high-efficiency, stable and low-cost method for preparing the α-aminobutyric acid.